Block copolymers have been used in the hot melt adhesive industry for a variety of applications. Block copolymers are often the preferred polymer base due to their good heat stability, high cohesive strength, and compatibility with a wide range of tackifiers and plasticizers.
Kraton G-1651, a high molecular weight S-EB-S linear A-B-A block copolymer, having a molecular weight of about 240,000, has been used in the past by the injection molding industry for shoe soles. Due to its high molecular weight, it is believed that it has not been previously used by the hot melt adhesive industry.
The use of hot melt adhesives comprised of conventional molecular weight block copolymers for purposes of adhering an absorbent article, such as a sanitary napkin, to an undergarment is known. Raykovitz et al., U.S. Pat. No. 4,704,110 teaches an absorbent article utilizing an adhesive with a S-B-S multi-block A-B-A-B-A-B copolymer. Nelson et al. EP 0,525,251, teaches an improved positioning adhesive including a S-I-S linear A-B-A block copolymer containing at least 25 parts styrene. Collins et al., U.S. Pat. No. 4,136,699 teaches a conventional molecular weight S-EB-S linear A-B-A block copolymer based construction and positioning adhesive for absorbent articles. The prior art, to date, has not shown the use of a high molecular weight S-EB-S in hot melt adhesives.
A positioning adhesive is commonly applied to a release liner and transfer coated to the garment facing surface of an article such as a sanitary napkin. Upon removal of the release liner, it is common for the adhesive to accidentally come in contact with itself prior to placement on the undergarment. This is particularly a problem with winged products. When the positioning adhesives' composition is in accordance with the teachings of Raykovitz, Nelson, or Collins, the positioning adhesive is of such high self adhesion (in excess of the strength of the garment facing surface) that the adhesive layers are separable only at the expensive of destroying the article. Thus there is a need for an adhesive which adheres adequately to an undergarment, yet can be separated from itself.
Another problem associated with prior art positioning adhesives concerns "stay in place" properties. Under conditions of low pressure, the positioning adhesive tends to lose adhesion to the undergarment. In many instances this loss of adhesion causes the sanitary napkin to shift from its original position or become unattached from the undergarment entirely.